Currently, the light source mainly includes HID (High-Density Discharge) lamp and LED (Light Emitting Diode). For increasing the efficiency in lighting, the inverter used for driving light source is improved gradually. The circuit architecture of the convention inverter is shown in FIG. 1. The piezoelectric inverter includes a filter unit 1, a pulse control unit 2, a power switch unit 3 and a power conversion unit 4. After a DC input power 91 passes through the filter unit 1, the pulse control unit 2 through controlling the conduction period of the power switch unit 3, and further controls the power conversion unit 4 to convert the period of the input power 91. Then, the power conversion unit 4 provides the driving power, and the pulse control unit 2 draws out a feedback signal from the power conversion unit 4. The power conversion unit 4 is further connected to an output coupling unit 5 for delivering energy to at least a lamp tube set 6. The circuit architecture described above purely utilizes the pulse control unit 2 to control the conduction or disconnection of the power switch unit 3, which is namely the conventional BOOST MODE technology, wherein the waveform of the power passing through the power switch unit 3 is a square wave which rises and falls steeply (as shown in FIG. 2). However, the conventional driving method is disadvantageous of damaging the inverter and the light source. Take HID lamp as an example. When operation, except the filling gas and the fluorescent powder are related to the working efficacy of HID lamp, the power provided by the inverter for driving the lamp which rises and falls steeply also might reduce the life time of the HID lamp. Moreover, in the inverter, except utilization of the traditional coil to produce electromagnetic induction, the piezoelectric blade is also used. But, the square wave of power might cause the output voltage of the piezoelectric blade to have excess undulation, so as to reduce the life time of the piezoelectric blade itself and also the loading. Therefore, the applicant previously disclosed TWP No. I256862, entitled “Method for controlling power source in modulation mode”, filed in Nov. 17, 2004, for improving the drawbacks of BOOST MODE control method as described above. In this patent, modulation energy with gradually changed amplitude is inserted between the conduction (ON) and the disconnection (OFF), so that the waveform of the driving power still can have a smaller voltage amplitude during the disconnection, and thus, the voltage amplitude difference between the conduction (ON) and the disconnection (OFF) becomes smaller, thereby reducing the impact on the piezoelectric blade and the loading. However, in this patent, when driving the loading in practice, owing to the added modulation energy, the falling of the waveform becomes smaller between the conduction (ON) and the disconnection (OFF), so that the brightness decrease of the lamp tube is limited due to the conversion characteristic of the piezoelectric blade, even the brightness of the lamp tube might become non-uniform. Therefore, there is a need to improve thereof.